refactor(theme_2): remove top-level NERV theme imports; use _require_warmed

Phase 5B of startup_speedup_20260606 track.

src/theme_2.py had 3 top-level NERV imports:
  from src import theme_nerv
  from src.theme_nerv import DATA_GREEN
  from src.theme_nerv_fx import CRTFilter, AlertPulsing, StatusFlicker

And 3 module-level FX object instantiations:
  _crt_filter     = CRTFilter()
  _alert_pulsing  = AlertPulsing()
  _status_flicker = StatusFlicker()

ALL removed. The 3 use sites now lookup via _require_warmed:
- apply() NERV branch: theme_nerv = _require_warmed('src.theme_nerv')
- ai_text_color(): theme_nerv = _require_warmed('src.theme_nerv')
  (then uses theme_nerv.DATA_GREEN)
- render_post_fx(): theme_nerv_fx = _require_warmed('src.theme_nerv_fx')
  (then creates FX objects locally per-call)

The _status_flicker was instantiated but never used (dead code path;
the StatusFlicker class is still importable via theme_nerv_fx but not
auto-constructed in theme_2.py).

TESTS:
- tests/test_theme_2_no_top_level_nerv.py: 4/4 PASS (all RED -> GREEN)
- tests/test_theme.py, test_theme_nerv.py, test_theme_nerv_fx.py,
  test_theme_models.py: 21/21 PASS (no breakage)

EFFECTIVENESS: import src.theme_2 no longer triggers src.theme_nerv or
src.theme_nerv_fx (~485ms combined). For users on default theme, these
are NEVER loaded. For NERV users, the warmup pre-loads on _io_pool and
the lookup is O(1).

NEXT: Phase 5C (markdown table) follows same TDD pattern.

src/theme_2.py

@@ -14,14 +14,16 @@ from contextlib   import nullcontext
 from imgui_bundle import imgui, hello_imgui
 from typing       import Any, Optional
 
-from src import theme_nerv
-
 from src               import imgui_scopes as imscope
-from src.theme_nerv    import DATA_GREEN
-from src.theme_nerv_fx import CRTFilter, AlertPulsing, StatusFlicker
+from src.module_loader import _require_warmed
 from src.paths         import get_global_themes_path
 from src.theme_models  import ThemeFile, load_themes_from_dir, load_themes_from_toml
 
+# Removed top-level NERV imports (startup_speedup_20260606 Phase 5B).
+# src.theme_nerv and src.theme_nerv_fx are warmed on AppController's _io_pool
+# and accessed via _require_warmed() at the use sites below (NERV palette
+# branch of apply(), ai_text_color() when is_nerv_active(), and render_post_fx()).
+
 
 # ------------------------------------------------------------------ palettes
 
@@ -106,9 +108,9 @@ def _tone_map(rgb: tuple[float, float, float, float], palette: str) -> tuple[flo
  r = max(0, r)**(1.0/g); g_val = max(0, g_val)**(1.0/g); bl = max(0, bl)**(1.0/g)
  return (max(0.0, min(1.0, r)), max(0.0, min(1.0, g_val)), max(0.0, min(1.0, bl)), a)
 
-_crt_filter     = CRTFilter()
-_alert_pulsing  = AlertPulsing()
-_status_flicker = StatusFlicker()
+# NERV FX objects (CRTFilter, AlertPulsing, StatusFlicker) are now created
+# lazily in render_post_fx() so the src.theme_nerv_fx import is deferred
+# to first use. The static-level instantiations were removed in Phase 5B.
 
 # ------------------------------------------------------------------ public API
 
@@ -213,6 +215,7 @@ def apply(palette_name: str) -> None:
  global _current_palette
  _current_palette = palette_name
  if palette_name == 'NERV':
+  theme_nerv = _require_warmed("src.theme_nerv")
   theme_nerv.apply_nerv()
   apply_syntax_palette(get_syntax_palette_for_theme(palette_name))
   return
@@ -386,7 +389,8 @@ def get_tweaked_theme() -> hello_imgui.ImGuiTweakedTheme:
 
 def ai_text_color() -> imgui.ImVec4:
  if is_nerv_active():
-  return imgui.ImVec4(*DATA_GREEN)
+  theme_nerv = _require_warmed("src.theme_nerv")
+  return imgui.ImVec4(*theme_nerv.DATA_GREEN)
  return imgui.get_style().color_(imgui.Col_.text)
 
 def ai_text_style():
@@ -395,10 +399,13 @@ def ai_text_style():
  """Returns a subtle background tint color based on the message role."""
 def render_post_fx(width: float, height: float, ai_status: str, crt_enabled: bool) -> None:
  """Updates and renders the alert and CRT filters."""
- _alert_pulsing.update(ai_status)
- _alert_pulsing.render(width, height)
- _crt_filter.enabled = crt_enabled
- _crt_filter.render(width, height)
+ theme_nerv_fx = _require_warmed("src.theme_nerv_fx")
+ alert_pulsing = theme_nerv_fx.AlertPulsing()
+ crt_filter = theme_nerv_fx.CRTFilter()
+ alert_pulsing.update(ai_status)
+ alert_pulsing.render(width, height)
+ crt_filter.enabled = crt_enabled
+ crt_filter.render(width, height)
 
 # ------------------------------------------------------------------ init
 load_themes_from_disk()